Square metres to lineal metres
Decking is sold by the lineal metre, but a deck is measured in square metres, so the first job is the conversion. Each row of boards covers its own width plus the gap beside it. Divide the deck dimension that runs across the boards by that pitch and you have the number of rows. Multiply rows by the board length and you have lineal metres.
lineal metres per m² = 1 ÷ ((board width + gap) ÷ 1000)
For a 90 mm board with a 4 mm gap the pitch is 94 mm, which is 10.64 lineal metres for every square metre. An 86 mm treated pine board at the same gap is 11.11. A 140 mm board is 6.94. That single number is the one to remember when a supplier quotes per lineal metre and you are standing there with an area.
| Board | Gap | lm per m² | 24 m² deck |
|---|---|---|---|
| 70 mm merbau | 4 mm | 13.51 | 324 lm |
| 86 mm treated pine | 4 mm | 11.11 | 267 lm |
| 90 mm hardwood | 4 mm | 10.64 | 255 lm |
| 136 mm composite | 5 mm | 7.09 | 170 lm |
| 140 mm hardwood | 4 mm | 6.94 | 167 lm |
Figures are before waste and before rounding up the last row.
Gaps and movement
Timber moves. Boards laid tight in winter will buckle in summer, and boards laid with a big gap in summer will open to a toe-catcher by August. A 3 to 5 mm gap suits most Australian hardwoods and treated pine, with the smaller gap for boards that arrive dry and the larger for boards still green from the yard. Composite boards expand along their length more than across it, and the manufacturer's spacer sets both gaps. Whatever the gap, keep it consistent, because the eye picks up a 2 mm difference from across the yard.
Joist spacing
Joist spacing is set by the board, not the joist. Nineteen millimetre softwood and 19 to 22 mm hardwood boards are normally laid over joists at 450 mm centres. Thicker hardwood, 32 mm and up, can go to 600 mm. Composite follows the manufacturer's span table and is often 400 mm for residential loads and tighter again for a diagonal pattern. Joists run across the boards, so the calculator counts them along the board direction and adds one for the far end. The joist size itself, and the bearers and posts under them, come from the span tables in AS 1684 or your engineer, and are not part of this calculation.
Fixings
Two screws at every joist, driven about 20 mm in from each edge. Outdoors that means stainless steel, or at minimum a Class 4 coated screw, because anything less will streak the boards with rust within a couple of seasons. For 19 to 22 mm boards a 50 to 65 mm screw is the usual. Hidden fixing systems for composite change the count, so follow the maker's figure if you are using clips.
Worked example
A 6 × 4 m deck with 90 mm hardwood boards at a 4 mm gap, boards running the 6 m way. The pitch is 94 mm, so 4 ÷ 0.094 = 42.6, which rounds up to 43 rows. That is 43 × 6 = 258 lineal metres, or 284 with 10 percent waste. In 5.4 m lengths that is 53 boards, with each row taking one full board and a 600 mm piece, so consider 6.0 m stock instead and save the joins. Joists at 450 mm along the 6 m run: 6 ÷ 0.45 = 13.3, so 14 spaces and 15 joists, each 4 m long. Screws: 43 rows × 15 joists × 2 = 1,290, so buy 1,355 or the next box up.
Common questions
How do I convert decking m² to lineal metres?
Divide the area by the board width plus the gap in metres. 90 mm boards with a 4 mm gap give 10.6 lineal metres per m².
How much waste for decking?
10 percent straight, 15 percent diagonal. Choose a stock length that suits the deck to cut joins and offcuts.
What joist spacing for decking?
450 mm centres for 19 to 22 mm timber. Composite per the manufacturer, often 400 or 450 mm.
How many screws per board?
Two at every joist. Stainless or Class 4, 50 to 65 mm long for 19 to 22 mm boards.
This calculator gives decking quantities from the dimensions you enter. Joist, bearer and post sizes are structural and should come from AS 1684 span tables, the product's own tables, or your engineer.